U.S. Pat. No. 4,027,265 discloses a QPSK demodulator for "unbalanced" but not unsymmetrical signals. "Unbalanced" in the reference means that the amounts f phase shift indicating data bits are not multiples of 90.degree..
The reference device, which is more complex than the instant device, reconstructs the reference carrier by attempting to correlate with phasors of the received signal. This is accomplished by separate dual correlators 58,62 whose outputs ultimately control the frequency/phase of VCO 88. Another set of dual correlators 38,48 recovers data streams 1 and 2.
By contrast, the present invention requires only one set of dual correlators (11,21) to capture the I and Q components of the signal "phasor". Each component then yields a data stream, the raw I and Q data, respectively. Filtering the channels in bit rate matched filters (13,23) followed by time alignment (32) and unequal amplification (at 12,22) produces a phase error signal which facilitates recovery of the VCO (30) control signal.
U.S. Pat. No. 4,079,329 discloses a digital demodulator for recovering a baseband signal from a selectable bit rate PSK modulated signal that is not unsymmetrical.
The reference device requires data synchronization simultaneously with, or prior to, carrier synchronization. This is different and more complex than in the present invention.
U.S. Pat. No. 4,085,378 discloses a QPSK demodulator in which the inphase and quadraphase channels are independently cross-coupled to minimize crosstalk.
U.S. Pat. No. 4,092,606 discloses a QPSK signal demodulator using a phase lock loop including a variable frequency coherent reference that is dithered at a low rate to derive a relatively low level tracking error phase for the lock loop.
U.S. Pat. No. 4,344,178 discloses a QPSK demodulator which uses a sample and hold detector in the phase detection circuit, which presupposes simultaneous synchronization (clock recovery) of data and carrier.
U.S. Pat. No. 4,559,499 discloses an AGC circuit for a symmetrical QPSK demodulator. It does not disclose the time alignment means 32, optimal (bit-rate-matched) filters 13,23, or unequal-gain amplifiers 12,22 of the present invention.
U.S. Pat. No. 4,581,586 discloses a QPSK demodulator in which the I channel data are first recovered, and then the Q channel data are recovered.